Porous silicon is found in a variety of applications where its benign nature, electronic and optical properties and entrapment of other materials is desirable.
There are numerous methods available for making porous silicon. For example, and as described in PCT/GB96/01863, bulk crystalline silicon can be rendered porous by partial electrochemical dissolution in hydrofluoric acid based solutions. This etching process generates a silicon structure that retains the crystallinity and the crystallographic orientation of the original bulk material. Hence, the porous silicon formed is a form of crystalline silicon. Broadly, the method involves anodising, for example, a heavily boron doped CZ silicon wafer in an electrochemical cell which contains an electrolyte comprising a 20% solution of hydrofluoric acid in an alcohol such as ethanol, methanol or isopropylalcohol (IPA). Following the passing of an anodisation current with a density of about 50 mA cm−2, a porous silicon layer is produced which may be separated from the wafer by increasing the current density for a short period of time. The effect of this is to dissolve the silicon at the interface between the porous and bulk crystalline regions.
Porous silicon may also be made using the so-called stain-etching technique which is another conventional method for making porous silicon. This method involves the immersion of a silicon sample in a hydrofluoric acid solution containing a strong oxidising agent. No electrical contact is made with the silicon, and no potential is applied. The hydrofluoric acid etches the surface of the silicon to create pores. Up until now stain etching has tended to have poor reproducibility, low yield, be strongly exothermic and produce internal surface areas which are significantly lower than those obtained from anodising wafers.
These existing stain etch methods, particularly for making very high porosity silicon, are not very efficient and they tend to be slow. Further, anodisation currently uses relatively expensive silicon wafers. Existing stain etch methods have not proved to be particularly suitable for producing fully porous silicon which also means the porosified silicon will not be fully biodegradable and will not all be converted to silicic acid in-situ. Existing attempts at fully porosifying silicon tend to result in the porous silicon retaining areas of non porous silicon, particularly at the centre of the silicon particle.
There is a continued need for alternative and preferably improved methods for making porous silicon which address at least some of the issues mentioned above.